(i) Field of the Invention
The present invention relates to a process for continuously preparing a methylene-crosslinked polyarylamine.
More specifically, the present invention is directed to a process for the continuous preparation of a methylene-crosslinked polyarylamine from aniline and formalin by the use of a multi-stage reactor, the aforesaid process being characterized by reacting aniline with formalin in the presence of hydrochloric acid under conditions that formalin is divided and placed in 3 or more stages; hydrochloric acid is used in a molar ratio of from 0.1 to 0.5 mole per mole of aniline; aniline is used finally in a molar ratio of from 1.5 to 4.0 moles per mole of formalin; reaction temperatures in the first stage, the second stage, the third et seq. stages and the final stage are adjusted to 20-50.degree. C., 40-70.degree. C., 50-90.degree. C. and 110.degree. C. or more, respectively; and molar ratios of water to aniline are 1.3-2.5 in the first stage, 1.9-5.0 in the second stage, and 2.4-5.7 in the third et seq. stages.
The methylene-crosslinked polyarylamine obtained by the present invention (hereinafter referred to as ["poly (MDA)"]will be reacted with phosgene to produce a polyisocyanate, and this polyisocyanate will be used as a raw material for polyurethane resins such as hard and semi-hard foams and elastomers.
The methylene-crosslinked polyarylamine can be represented by the formula (I) ##STR1## wherein n is 0 or more.
(ii) Description of the Related Art
In general, it is known that a polyisocyanate prepared from the above-mentioned raw materials is less reactive, when the content of a hydrolyzable chlorine compound is high.
In case that the polyisocyanate containing a large amount of the hydrolyzable chlorine compound is polymerized with a polyamine or a polyol and used as a polyurethane resin, a reaction rate is low, and thus it is utilized as a known means to increase the amount of a catalyst for urethane production such as a tertiary amine or an organic metal for the acceleration of the reaction rate. In this case, a curing rate can be increased, but unpreferably, a bodying speed is also accelerated, with the result that a pot life shortens and an expansion ratio decreases.
Therefore, the polyisocyanate containing a less amount of the hydrolyzable chlorine compound is very useful as the raw material of the urethane, and it is preferred that the content of the hydrolyzable chlorine compound is 0.18% or less in terms of chlorine (the same shall apply hereinafter).
Japanese Patent Publication No. 50-38095 discloses that a polyamine containing a large amount of an amino group can be continuously prepared by the use of a multi-stage reactor in which each of the stages is adjusted in a certain temperature range, and that formalin is divided and placed in the two or more stages so as to change a molar ratio of the raw material amine to formalin in the respective stages, whereby the distribution of polyamine nuclei can be controlled.
However, this publication does not have the description regarding how to decrease the content of the hydrolyzable chlorine compound. In a tracing test of the present inventors, a continuous reaction was carried out under conditions that formalin was divided and placed in two stages, the molar ratio of aniline/formalin was 1.8, the molar ratio of hydrochloric acid/aniline was 0.5, and the reaction temperature of the last reaction stage was adjusted to 100.degree. C, and as a result, the content of N-methylmethylenedianiline (hereinafter referred to as "N-methyl-MDA") was 0.35%. Furthermore, this N-methyl-MDA was reacted with phosgene to produce a polyisocyanate. In this case, the hydrolyzable chlorine compound in the thus produced polyisocyanate contained 0.42% of chlorine. Accordingly, the problem to be solved is to decrease the amount of a precursor which will change into the hydrolyzable chlorine compound in the poly(MDA) in the multi-stage reaction.